Field of the Invention
The present invention relates to an aircraft window having an electromagnetic shielding function, and a closing member for an opening portion having an electromagnetic shielding function.
Description of the Related Art
During a cruising flight, a landing or a takeoff, an aircraft is required to be able to fly safely without occurrence of malfunctions or unforeseen behaviors (up-set) to the aircraft in high intensity radiated fields (HIRF), which represent electromagnetic environments from radios, televisions, radars, transmitters, and other sources. For this reason, it is necessary to take HIRF protection measures as required in (14CFR) Sections 23.1308, 25.1317, 27.1317, and 29.1317, High-intensity Radiated Fields (HIRF) protection, which stipulate Regulations (airworthiness requirements) of FAA (Federal Aviation Administration).
Recently, the importance of the protection of electric/electronic systems of aircraft has been significantly increasing for the following reasons:
1) a greater dependence on electric/electronic systems that execute functions required for continued safe flight and landing of the aircraft;
2) a decrease in electromagnetic shielding by certain types of composite materials (fiber reinforced resins) used for designing the aircraft;
3) an increase in the susceptibility (sensitivity) of the electric/electronic systems to HIRF along with the increase in operating rates of data buses or processors, the higher densities of ICs (Integrated Circuits) and cards, and the higher sensitivity of electronics;4) an expansion of a usage frequency particularly to a high-frequency band of 1 GHz or higher;5) an increase in the severity of HIRF environments along with increases in the number of RF transmitters and electric power; and6) an adverse effect on some aircrafts during exposure to HIRF.
Meanwhile, inside an aircraft, radio waves or electromagnetic waves (hereinafter, both as “electromagnetic waves”) from various electronics such as mobile telephones, game machines, and notebook computers, and PEDs (Personal Electro Devices) such as active-type RFID (Radio Frequency Identification) tags attached to air cargos may have adverse effects on, for example, communications with airport control towers, and communications or controls of navigation for a flight along a predetermined route. Thus, as is well known, passengers are asked to refrain from using various electronics inside the aircraft.
Electromagnetic waves invade an airframe of the aircraft, e.g., a cabin (a seat space), a cockpit (a flight deck), and an avionics bay through a window provided in an opening portion formed in the airframe.
To prevent the invasion of the electromagnetic waves, an electromagnetic shield film is held between a plurality of window panels constituting the window (e.g., see JP 2003-523911A (Translation of PCT Application)).
As shown in FIG. 10, the aircraft window generally includes a window body 21, a window frame 30 that surrounds an outer peripheral portion of the window body 21, and a fixing member, such as a clamp 28, that fixes the window body 21 to the airframe via the window frame 30. The window body 21 has a structure in which window panels 22 and 23, and an electromagnetic shield layer 25 are laminated.
A gasket 50 (an interposition member) for ensuring airtightness is held between the window body 21 and the window frame 30.
A rubber-based material, such as EPDM rubber (ethylene-propylene-diene rubber) and silicone rubber, used for a typical gasket is a non-conductive material, and thus has no electromagnetic shielding effect. Therefore, the non-conductive gasket functions as if it were a slot through which electromagnetic waves pass. Electromagnetic waves in a band of high frequencies whose ½ wavelength is sufficiently smaller than the opening width of the slot invade the airframe through the slot between the window body 21 and the window frame 30 without being attenuated.
For this reason, a method of forming a gasket by a material provided with conductivity by mixing a filler made of metal, carbon or the like, into a rubber-based material has been proposed. The applicant of the present invention also filed a patent application regarding the gasket 50 having conductivity, and the patent application was granted (Japanese Patent No. 5060647, which corresponds to U.S. Pat. No. 8,998,140).
In Japanese Patent No. 5060647, and accordingly U.S. Pat. No. 8,998,140, conductive paint 43 is applied to outermost peripheral portions of the window panels 22 and 23, and the outer periphery of the electromagnetic shield layer 25 exposed from between the window panels 22 and 23 is brought into conduction with the gasket 50 through a planar interface by the conductive paint 43 (see FIG. 10). The electromagnetic shield layer 25 is grounded (bonded) to the airframe through the gasket 50, the fixing member such as the clamp 28, and the window frame 30.
Airtightness required for the window is ensured by the gasket being compressed and deformed between the window body and the window frame.
However, when the amount of the conductive filler mixed into the rubber-based material of the gasket is increased so as to increase the conductivity of the gasket and thereby obtain a sufficient electromagnetic shielding effect, the gasket loses flexibility (elasticity) required for securing the airtightness.
Therefore, at present, there is no choice but to keep the lower limit of the volume resistivity of the conductive gasket to, for example, 5 Ωcm. It is difficult for the conductive gasket to sufficiently attenuate electromagnetic waves.
For example, an electromagnetic wave entering the window panel 22 at an angle indicated by arrow L1 in FIG. 10 is reflected by the electromagnetic shield layer 25 to pass through the gasket 50, and then reflected by the window frame 30 to pass through the gasket 50 again. If the attenuation effect from an absorption loss by the gasket 50 is small, the electromagnetic wave is not sufficiently attenuated, and enters the airframe.
An electromagnetic wave traveling along arrow L2 at an incident angle shallower than that indicated by L1 toward a corner portion C between an end surface of the window panel 22, to which the conductive paint 43 is applied, and an inclined surface thereof also invades the airframe if the electromagnetic wave is not sufficiently attenuated when passing through the gasket 50.
In view of the problems as described above, an object of the present invention is to provide an aircraft window and a closing member provided in an opening portion similarly to the aircraft window, which enable to ensure sufficient electromagnetic shielding performance even if a gasket (an interposition member) cannot be provided with conductivity required for sufficiently attenuating electromagnetic waves, or even if the interposition member has no conductivity.